1. Field of the Invention
The present invention relates to a power-receiving device, a wireless power-feeding system including the power-receiving device, and a wireless communication system including the power-receiving device.
2. Description of the Related Art
Various electronic appliances have spread and a variety of products is shipped to the market. In recent years, the spread of portable electronic appliances such as cellular phones and digital video cameras is apparent.
A cellular phone or a digital video camera has a built-in battery serving as a power storage means. Currently, such a battery is charged, in most cases, by bringing it in direct contact with a household AC power source serving as a power-feeding means. In view of this, research and development of methods of wirelessly charging batteries or feeding electricity to loads for improved convenience have advanced. Typical examples of methods for wireless power-feeding systems include the electromagnetic coupling method (also called electromagnetic induction method), the radio wave method (also called micro wave method), and the resonance method (also called magnetic resonance method).
A wireless power-feeding system using the electromagnetic coupling method cannot yield high transmission efficiency when a power-feeding coil in a power-feeding device and a power-receiving coil in an electronic appliance are displaced. Accordingly, a power-feeding device equipped with a plurality of power-feeding coils, and a technique to move the power-feeding coil so that it can be aligned with the power-receiving coil have been developed (see Patent Document 1, for example).
Wireless power-feeding systems using the resonance method have attracted attention and their research and development have been promoted because they yield high transmission efficiency, for middle and long distance use (see Patent Document 2, for example).
On the other hand, more and more portable electronic appliances such as recent cellular phones and smartphones function as wireless IC cards having applications such as electronic money. Since wireless IC cards need a wireless communication function, these portable electronic appliances have a built-in antenna for communication (see Patent Document 3, for example).